1. Field of Invention
The invention relates to carbon monoxide selective oxidizing catalysts that promote a reaction of oxidizing carbon monoxide by giving carbon monoxide priority over hydrogen and manufacturing methods for the same.
2. Description of Related Art
A reformed gas obtained by reforming gasoline or a hydrocarbon-based fuel such as natural gas or alcohol is employed as a hydrogen-rich fuel gas used in an electrochemical reaction in a fuel cell. Such a reformed gas generally contains a certain amount of carbon monoxide. However, in a fuel cell provided with a platinum-based catalyst such as a proton-exchange membrane fuel cell, if the supplied gas contains carbon monoxide, the catalyst may be poisoned by carbon monoxide and the cell performance may deteriorate. Therefore, a carbon monoxide concentration reduction apparatus is employed to reduce the concentration of carbon monoxide contained in the reformed gas before the reformed gas is supplied to the fuel cell, thereby preventing the catalyst in the fuel cell from being poisoned by carbon monoxide.
One of the reactions performed in the carbon monoxide concentration reduction apparatus is a carbon monoxide selective oxidizing reaction that oxidizes carbon monoxide by giving carbon monoxide priority over hydrogen. The oxidation reaction of carbon monoxide is represented by an equation I described below.CO+(½)O2→CO2  I
Various types of catalysts that promote such a carbon monoxide selective oxidizing reaction are known. For example, Japanese Patent Application Laid-Open Publication No. 11-347414 discloses a catalyst in which a platinum alloy is supported on a carrier including mordenite, a kind of zeolite. According to the art disclosed in this publication, the percentage of metals other than the platinum contained in the platinum alloy is controlled to 20 to 50 atomic percent, thereby improving the performance of the carbon monoxide selective oxidizing catalyst.
However, even in a case where the above-mentioned carbon monoxide selective oxidizing catalyst is used, the performance of reducing the concentration of the carbon monoxide contained in the fuel gas supplied to the fuel cell may be insufficient. Therefore, it is desired to obtain a carbon monoxide selective oxidizing catalyst with much superior performance of reducing the concentration of the carbon monoxide.
Generally, activity of the reaction promoted by catalysts can be improved by increasing the reaction temperature. On the other hand, the above-mentioned carbon monoxide selective oxidizing reaction is to selectively oxidize an extremely small amount of carbon monoxide present in a large amount of hydrogen. Therefore, in general, as the reaction temperature becomes higher, the oxidizing reaction of hydrogen becomes more active, thereby decreasing the efficiency of reducing the carbon monoxide concentration. This results in a problem that the fuel utilization efficiency decreases. In addition, as the reaction temperature becomes higher, undesirable reactions (for example, a methanation reaction that methanates hydrogen) become more active to consume more hydrogen, in addition to the carbon monoxide selective oxidizing reaction. This causes a problem that the fuel utilization efficiency decreases. Therefore, to improve performance of the entire fuel cell system, it is desirable that the carbon monoxide selective oxidizing catalyst should achieve a higher carbon monoxide concentration reduction rate, provide higher selectivity in oxidizing carbon monoxide, and exhibit sufficient activity at lower temperatures.